The goal of my research is to gain a better understanding of human psychopathology through the study of biochemical and behavioral variation in lower animals. Presently this objective is being pursued through a study of normal variation in levels of monoamine oxidase (MAO) activity in the laboratory mouse. Brain MAO activity was measured in the brains of female and male animals of a genetically heterogenous strain at 100, 200, 300 and 400 days old using PEA, 5-HT and tyramine as substrates. Brain MAO activity remained constant across all age groups when PEA and 5-HT were used as substrates but a sharp decrease in tyramine deamination was observed in the brains of 400 day old subjects. This variation in activity may be due to deterioration of the enzyme binding site, loss of tyramine affinity due to inhibition or competition from increases in endogenous substances in the brain with aging. Different levels of MAO activity has been demonstrated in the brain homogenates from selected inbred strains of mice using PEA, 5-HT and tyramine as substrates. The possibility of a classical Mendelian pattern of inheritance is being studied in F1, F2 and backcross generations using inbred lines selected from the previous study.